When one deletes a friend from Facebook because he or she knows that the Facebook friend in question would eventually delete them instead. Preemptive unfriending thus robs the deleted Facebook friend of the joy of being able to delete them.

John: Why'd you delete Bec from you friend list?
Adam: I knew she'd eventually delete me so I wanted to take the satisfaction of unfriending for myself.
John: Ah, a preemptive unfriending.